The United Nations migration agency said it was "deeply concerned" about some 260 Somali and Ethiopian refugees allegedly held and mistreated by criminal gangs in Libya, saying it believed a harrowing video of them posted on social media was authentic.

The International Organization for Migration noted the video posted June 9 on Facebook, in which it said many "abused Somalis and Ethiopians are seen huddled fearfully in a concrete room".

The IOM said a Somali journalist based in Turkey recorded the video call from a gang in which some refugees claimed to have been beaten. Some alleged having their teeth pulled out and arms broken.

The authenticity of the video could not be independently verified.

The agency said some captives' relatives had received videos asking them to pay $8,000 to $10,000 "or their child or relative will be killed". The relatives' exact location was not known, but the IOM said "the relevant authorities in this situation have been informed".

The IOM has long decried risks taken by human traffickers with the migrants and refugees they ferry through relatively lawless Libya and into the Mediterranean Sea by boat en route to Italy.

Libya has been without a stable, central government since the fall of Moammar Gadhafi in 2011 and has been a major departure point for tens of thousands of people, mostly from the Horn of Africa, who seek to cross the Mediterranean to reach relative peace and economic opportunity in Europe.

In the video posted on Facebook, a man calling himself Abdinajib Mohamed speaks to the camera: "I am here for a year now. I am in trouble. I am starved. Anyone who has gone through such ordeal would have hated life altogether. Look at my body, they beat me every day with batons. They don't want to release me."

Another young man who called himself Nur Ali Awale said he had been held for 15 months.

"They beat me with iron bars," he said. "I travelled from Ethiopia. They ordered me to pay $8,300, and my family cannot afford to pay that amount."

A veiled woman who said she had traveled from Bossaso city in northern Somalia with her two children said she had been beaten daily.